1. Field of the Invention
The present invention relates to a process for producing an aromatic polysulfone molding compound improved in mold-release characteristics.
2. Description of the Prior Art
Aromatic polysulfone resins are in use as engineering plastics superior in heat resistance, strength, stiffness, creep resistance, and flame retardance for electric and electronic components as well as for office automation equipment parts, automobile parts, etc. Recently, technical progress in the electronic field has been remarkable, electronic components, e.g. relays, switches, connectors, sockets, and coil bobbins, have been miniaturized, made thinner in wall, or sophisticated, and their high dimensional accuracy and high heat resistance have been required.
Aromatic polysulfone resins show low mold shrinkage because they are amorphous. The deterioration of properties such as strengths and modulus of elasticity are less, up to higher temperature, than those of high temperature resistant resins like polyphenylene sulfide and polyetherketone, because the glass transition points of aromatic polysulfone resins are higher than those of said high temperature resistant resins. Hence, aromatic polysulfone resins are materials fitted for electronic components which are required to have high dimensional accuracy and high heat resistance. In particular, the incorporation of a fibrous material such as glass fiber, silica-alumina fiber, or wollastonite into aromatic polysulfone resins further lowers their mold shrinkage and improves their strengths and modulus of elasticity, making them more fitted for electronic components. However, injection molding of aromatic polysulfone resins requires high pressure because of their relatively high melt viscosities and this, jointly with their low mold shrinkage, makes it very difficult to release their molded parts from mold cavities when electronic parts of small sizes and complicate shapes or those having thin-wall portions are injection molded, where sufficient draft cannot be taken. Hence, none of products with high dimensional accuracy can be molded in such cases. This is also quite true in the case of blends of aromatic polysulfone resins with such fibrous fillers as mentioned above.
A known means for solving this problem is to add metal soaps to aromatic polysulfone resins. For example, Japanese Patent Application Kokai (Laid-Open) No. 63-264667 proposes a molding compound produced by melt mixing 100 parts by weight of an aromatic polysulfone resin with 0.02-5 parts by weight of a metal soap. Although superior in mold release characteristics, this molding compound involves a problem in that the addition of metal soap causes considerable deteriorations of mechanical properties, such as tensile strength, flexural strength, and Izod impact strength, of moldings produced.
Japanese Patent Application Kokai (Laid-Open) No. 60-149629 also proposes the method of sticking 200-1000 ppm of a metal soap on the surface of pellets or particles of an aromatic polysulfone resin and subjecting the resulting composition to injection molding. In this case, the metal soap does not adversely affect the performance characteristics of the moldings, since the amount of metal soap is less than that in the molding compound of the former patent application and the soap, having heat history only once, undergoes practically no change such as decomposition. However, this method has a drawback in that at the time of molding, these pellets slip and hence the chargeability of pellets is much worse, thus stable molding being impossible.